Calibration system for a wavelength selective switch

ABSTRACT

Described herein is a calibration system (25) for a wavelength selective switch (1). The switch (1) is adapted for dynamically switching optical beams (5, 7) along respective trajectories between input and output ports disposed in an array (3) using a reconfigurable Liquid crystal on silicon (LCOS) spatial light modulator device (17). The calibration system (25) includes a monitor (27) for projecting an optical monitor beam (29) through at least a portion of the switch (1) onto the LCOS (17) and detecting the monitor beam (29) reflected from the LCOS (17). In response, system (25) provides a calibration signal (33) to an active correction unit (35) for applying a correction to one or more of the trajectories while maintaining a constant switching state in the LCOS (17).

CROSS-REFERENCE TO RELATED APPLICATION

This application is a 35 U.S.C. § 371 application based onPCT/US15/18322 filed Mar. 2, 2015, entitled “A Calibration System for aWavelength Selective Switch” which claims priority to U.S. ProvisionalPatent Application Ser. No. 61/947,991, filed Mar. 4, 2014, entitled “ACalibration System for a Wavelength Selective Switch”. The entiredisclosures of PCT Patent Application PCT/US15/18322 and U.S.Provisional Patent Application Ser. No. 61/947,991 and are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to optical switching devices and inparticular to a calibration system for a wavelength selective switch.While some embodiments will be described herein with particularreference to that application, it will be appreciated that the inventionis not limited to such a field of use, and is applicable in broadercontexts.

BACKGROUND

Any discussion of the background art throughout the specification shouldin no way be considered as an admission that such art is widely known orforms part of common general knowledge in the field.

Liquid crystal on silicon (LCOS) based wavelength selective switches(WSS) have been developed based on highly stabilized optical trains. Ina push for minimizing the height (profile) of WSS components(single-slot applications) there is a requirement to reduce thestability and thermal shielding of the components.

Previous known approaches introduce an optical inversion into theoptical system to maintain the required stability over temperature andmechanical aberrations. However, these approaches have not been able tobe scaled to higher functionality devices such as dual devices or highport count devices.

One suggested approach is to look at using the ability of the LCOSdevice to readjust and steer the light to overcome the change inalignment. However, this is problematic because it changes the image ofthe LCOS, giving rise to problems such as:

-   -   Transient changes to the port isolation which can be poorly        controlled.    -   Difficulties in ensuring calibrations are valid across all        operating conditions.    -   Significant variations in the orders of the switching and        attenuation images occur as alignment changes, so an        uncontrolled order may emerge as a port isolation issue.

SUMMARY OF THE INVENTION

It is an object of the invention, in its preferred form to provide animproved or alternative calibration system for a wavelength selectiveswitch.

In accordance with a first aspect of the present invention there isprovided a calibration system for a wavelength selective switch, theswitch being adapted for dynamically switching optical beams alongrespective trajectories between input and output ports disposed in anarray using a reconfigurable spatial light modulator device, the systemincluding:

-   -   a monitor for projecting an optical monitor beam through at        least a portion of the wavelength selective switch onto the        spatial light modulator and detecting the monitor beam reflected        from the spatial light modulator device and, in response,        providing a calibration signal; and    -   an active correction unit responsive to the calibration signal        for applying a correction to one or more of the trajectories.

Preferably, the same correction is applied simultaneously to each of therespective trajectories. The correction preferably includes a predefinedspatial offset of the optical beams at the output ports. The spatialoffset of the optical beams is preferably in a switching dimension beinga direction of switching by the spatial light modulator device.

The wavelength selective switch preferably includes a wavelengthdispersive element for spatially dispersing the optical beams in adispersion dimension and wherein the spatial offset of the optical beamsis in the dispersion dimension.

The active correction unit preferably provides control to a beamswitching module including:

-   -   an electrically controllable mirror tiltable at a number of        predefined angles; and    -   an optical power element having a focal length f and being        positioned at a distance f from both array and the mirror to        convert the angular correction of the trajectories to a        corresponding spatial offset in a dimension of the array.

The active correction unit preferably includes a control for the spatiallight modulator for selectively modifying a background slope image whilemaintaining the current switching state.

The wavelength dispersive element is preferably a grism and the activecorrection unit includes a temperature controller for controlling thetemperature of the grism.

The monitor beam is preferably projected from a first monitor port inthe array and received in a second monitor port in the array. The firstand second monitor ports are preferably the same port. The monitor beamis preferably projected onto a predetermined region of the spatial lightmodulator.

The monitor preferably includes:

-   -   a light source for directing the monitor beam onto the        predetermined region;    -   a controller for electrically controlling cells within the        predetermined region to selectively direct the monitor beam        along a monitoring trajectory relative to a switching        trajectory; and    -   a detector for detecting the optical power of the beam directed        along the monitoring trajectory.

The light source preferably produces a monitor beam having apredetermined wavelength. The light source is preferably a wavelengthlocked laser. The light source preferably includes a tunable element forselectively defining the predetermined wavelength of the monitor beam.The tunable element is preferably a Fabry-Perot etalon.

The controller is preferably adapted to:

-   -   apply first and second ramp steering images across at least a        subset of the cells within the predetermined region to reflect        the optical monitor beam at respective first and second angles        to the detector; and    -   upon detection of the optical power levels of the optical        monitor beam at the first and second angles by the detector,        produce the monitor beam based on the difference in the optical        power levels.

The predetermined region is preferably located in a peripheral region ofthe spatial light modulator. The spatial light modulator is preferablyan LCOS device.

The active correction unit preferably applies the correction to the oneor more trajectories while maintaining a constant switching state in thespatial light modulator.

In accordance with a second aspect of the present invention, there isprovided a calibration method for a wavelength selective switch, theswitch being adapted for dynamically switching optical beams alongrespective trajectories between input and output ports disposed in anarray using a reconfigurable spatial light modulator device, the methodincluding:

-   -   projecting an optical monitor beam through at least a portion of        the wavelength selective switch onto the spatial light        modulator;    -   detecting the monitor beam reflected from the spatial light        modulator device and, in response, providing a calibration        signal; and    -   in response to the calibration signal, applying a correction to        one or more of the trajectories while maintaining a constant        switching state in the spatial light modulator.

In accordance with a third aspect of the present invention, there isprovided a monitor device for a wavelength selective switch, the switchbeing adapted for dynamically switching optical beams along respectivetrajectories between input and output ports disposed in an array using areconfigurable liquid crystal spatial light modulator device, themonitor including:

-   -   a monitor for monitoring predetermined characteristics of one or        more of the optical beams; and    -   an active feedback controller responsive to the monitor for        simultaneously correcting for more than one of the beam        alignment, wavelength position and liquid crystal optical        flicker.

In accordance with a fourth aspect of the present invention, there isprovided a monitoring method for a wavelength selective switch, theswitch being adapted for dynamically switching optical beams alongrespective trajectories between input and output ports disposed in anarray using a reconfigurable liquid crystal spatial light modulatordevice, the method including:

-   -   monitoring predetermined characteristics of one or more of the        optical beams; and    -   in response to the monitoring, simultaneously correcting for        more than one of the beam alignment, wavelength position and        liquid crystal optical flicker.

In accordance with a fifth aspect of the present invention, there isprovided an optical system including:

-   -   one or more input ports for projecting input optical beams into        the system;    -   a spatial light modulator including a plurality of cells, each        cell being independently electrically drivable at one of a        number of predefined states for, in conjunction with other        cells, diffracting the optical beams into at least a zero        diffraction order and a higher diffraction order and selectively        steering the diffraction orders along predetermined        trajectories;    -   a monitor for detecting the trajectory of one or more        diffraction orders; and    -   one or more output ports for receiving predetermined diffraction        orders.

The spatial light modulator is preferably responsive to a monitor signalissued by the monitor for selectively adjusting the trajectory of one ormore diffraction orders of a first optical beam with respect to adiffraction order of another optical beam.

The spatial light modulator is preferably responsive to a monitor signalissued by the monitor for collocating a diffraction order for more thanone optical beam at the one or more output ports.

In accordance with a sixth aspect of the present invention, there isprovided an optical fiber mount including:

-   -   a base having a two dimensional upper surface; and    -   a plurality of v-shaped grooves disposed in the upper surface of        the base, each adapted for receiving an optical fiber, the        grooves being spaced apart in a first dimension and extending        parallel to each other in a second dimension perpendicular to        the first dimension;    -   wherein the grooves are symmetrically disposed about a central        axis in the first dimension and the spacing between        predetermined fiber pairs in the first dimension is different.

A first subset of the grooves is preferably spaced apart in the firstdimension by a first distance and a second subset of the grooves isspaced apart in the second dimension by a second distance. The firstdistance is preferably 250 μm. The second distance is preferably 375 μm.

The fiber mount preferably includes twenty grooves for receiving up totwenty optical fibers. The central axis is preferably situated 500 μmfrom the nearest neighboring fibers.

In accordance with a seventh aspect of the present invention, there isprovided a calibration system for monitoring the operation of apixelated optical phased array structure, the system including:

-   -   a first laser for projecting a reference signal onto a portion        of the pixelated optical phased array structure,    -   a sensor for monitoring the return reference signal therefrom,    -   a control system for adjustment of position of phase patterns        based on the sensed return signal.

The pixelated optical phased array structure preferably forms part of anoptical system having a series of input/output ports, an opticaldispersion device and optical power elements and wherein an input signalfrom the input/output ports is projected through each of the opticaldispersion device and optical power elements.

The light signal preferably follows a path through the optical elementsrelative to the input signal.

The input signal is preferably incident onto the pixelated opticalphased array structure in a first region and the reference signal isincident onto the pixelated optical phased array structure in a secondregion separate to the first region. The first laser is preferablytunable.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the disclosure will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 is a schematic perspective view of a WSS according to anembodiment of the invention;

FIG. 2 is a schematic functional view of the WSS of FIG. 1 showingcommunication between a calibration system and other components withinthe WSS;

FIG. 3 is a schematic functional view of a monitor device of acalibration system within the WSS of FIGS. 1 and 2;

FIG. 4 is a schematic plan view of a beam switching module of the WSS ofFIG. 1;

FIG. 5 illustrates a process flow of steps in a method for calibratingoptical beams in the WSS of FIGS. 1 and 2;

FIG. 6 illustrates a process flow of the steps performed by the WSS ofFIGS. 1 and 2 for calibrating optical beams;

FIG. 7 is a state transition diagram describing the high level flowbetween the steps of FIG. 6;

FIG. 8 illustrates the process flow of the initialization process ofFIG. 6;

FIG. 9 illustrates the process flow of the MEMS mirror alignmentprocedure of FIG. 6;

FIG. 10 illustrates the process flow of the LCOS wavelength referenceprocedure of FIG. 6; and

FIG. 11 is a schematic front view of a fiber v-groove array formaintaining a plurality of optical fibers in a predetermined array.

DETAILED DESCRIPTION

System Overview

Preferred embodiments of the present invention have been developed foruse in dual source wavelength selective switch (WSS) devices. Anexemplary WSS device incorporating two independent optical sources willinitially be described with reference to FIG. 1. However, it will beappreciated that the embodiments described herein are applicable toother types of WSS devices such as single source devices and even, insome cases, to other types of optical switches.

Referring initially to FIG. 1, WSS device 1 includes an array 3 ofoptical fibers comprising both input optical fibers (common ports) forprojecting input optical beams 5 indicative of wavelength divisionmultiplexed optical signals and output optical fibers (add/drop ports)for receiving output optical beams 7 indicative of individual wavelengthchannels. Array 3 is divided into fibers sourcing beams from twodifferent independent optical sources: Source A and Source B. Ends offibers in array 3 are mounted in a fiber v-groove array 9 and projectrespective optical beams through corresponding micro-lenses of amicro-lens array 11 disposed opposite the fiber ends. Micro-lenses ofarray 11 are cylindrical or spherical or a series combination ofcylindrical and spherical in profile and act to define a beam waist ofthe initially diverging beams emerging from fibers of array 3. In otherWSS devices, different combinations of input and output fiber ports areused depending on the particular application.

A front end 13 of WSS 1 includes various components for preprocessingthe input optical beams from both sources including polarization controland beam alignment. A diffractive grism 15 spatially separatesindividual wavelength channels from the input optical beams in adispersion plane (x-y plane in FIG. 1). A reconfigurable liquid crystalon silicon (LCOS) spatial light modulator 17 includes a two dimensionalarray of individually electrically drivable pixels or cells 19 and isconfigured to individually processes the wavelength channels todynamically switch the channels along respective trajectories in aswitching plane (x-z plane in FIG. 1) between the input and outputfibers. Curved mirror 21 and lens 23 provide appropriate focusing andcollimation of the beams throughout WSS 1 in a similar manner to thatdescribed in U.S. Pat. No. 7,092,599 to Frisken entitled “WavelengthManipulation System and Method” and assigned to Finisar Corporation.

Referring to FIG. 2, WSS 1 includes a locked calibration system 25 fordynamically adjusting the alignment of the optical trajectories withrespect to the optical fibers to correct for optical misalignments dueto thermal and mechanical instabilities.

With reference to both FIGS. 1 and 2, locked calibration system 25includes a monitor 27 for projecting an optical monitor beam 29 throughWSS 1 onto LCOS 17 and for detecting monitor beam 29 reflected from LCOS17. Monitor beam 29 is projected from a first monitor fiber 31 in oradjacent array 3 and received at the same fiber upon reflection fromLCOS 17. In another embodiment, monitor beam 29 is projected from afirst fiber and received at a second fiber that is separate to the firstfiber. In response to the received monitor beam, monitor 27 provides acalibration signal 33 to an active correction unit 35. In response tothe received calibration signal 33, unit 35 calculates an appropriatecorrection to one or more of the trajectories in one or both of theswitching plane and the dispersion plane. This correction is applied bytransmitting a correction signal to one or more optical elements withinWSS 1 while maintaining a predefined switching state in LCOS 17. Apredefined switching state relates to a predefined LCOS steering imageor function defined by states of cells within a region of the LCOS tosteer an optical beam from one fiber to another specific fiber. The oneor more elements for which the correction can be applied include grism15, LCOS 17 and an electrically controllable micro-electromechanicalmirror (MEMS) that is located within front end 13. The MEMS mirror isdescribed in detail below.

Referring to FIG. 3, monitor 27 includes a light source in the form of awavelength locked distributed feedback laser 39 for directing monitorbeam 29 onto a predetermined reference region 41 of LCOS 17. Referenceregion 41 is preferably located in a peripheral region of LCOS 17 so asto avoid interference with the routine switching of the wavelengthchannels, which typically occurs in a central region. In anotherembodiment, reference region 41 corresponds to an unused channel regionon LCOS 17. Reference region 41 encompasses a plurality of liquidcrystal cells 19 and is similar in shape and size to other wavelengthchannel regions on LCOS 17. However, in other embodiments, referenceregion 41 is smaller or larger than other wavelength channel regions.Beam 29 emitted by laser 39 follows an optically similar path as thewavelength channels being switched, but is incident onto LCOS 17 inreference region 41.

In other embodiments, other types of laser are used including tunablesemiconductor lasers. Referring still to FIG. 3, monitor 27 alsoincludes a controller 43 for electrically controlling cells withinreference region 41 to selectively direct monitor beam 29 along amonitoring trajectory relative to the switching trajectories and back tomonitor fiber 31. A detector 45, including at least one photodiode,detects the optical power of monitor beam 29 directed along themonitoring trajectory and feeds this optical power data back tocontroller 43.

Laser 39 produces beam 29 having a predetermined reference wavelengthdefined by a wavelength discriminating element in the form of aFabry-Perot etalon (not shown).

Referring again to FIG. 2, locked calibration system 25 includes an LCOScontrol line 47 for providing a first feedback control signal fromactive correction unit 35 to LCOS 17. The first feedback control signalselectively modifies a background slope image applied to cells 19 ofLCOS 17, while maintaining the current LCOS switching state. Forexample, for a simple ramp function the position of each of the resetpoints in the switching dimension remains fixed while allowingvariations in the phase strength from top to bottom. For context, if theimage could be viewed as a grayscale image, a dark-to-light orlight-to-dark fading from top to bottom of the screen would be added fora particular region while the overall image structure is kept unchanged.For a phase image this allows very small variations of steering of abeam of light without creating discontinuities in the beam coupling orany of the port isolation terms.

Locked calibration system 25 also includes a temperature control line 49for sending a second feedback control signal from active correction unit35 to a temperature controller (not shown) for grism 15. The secondcontrol signal selectively controls the temperature of grism 15, whichdetermines the channel wavelength centering in the dispersion plane.

Locked calibration system 25 also includes a switching feedback controlline 51 for providing feedback from active correction unit 35 to a beamswitching module 53 disposed within front end 13. As illustrated in FIG.4, module 53 includes an electrically controllable MEMS 55 that iselectronically tiltable at a number of predefined angles in theswitching plane (x-z plane in FIG. 4) to selectively direct the opticalbeams in a similar manner to that described in PCT ApplicationPublication WO/2014/015129 to Frisken entitled “Polarization DiverseWavelength Selective Switch” and assigned to Finisar Corporation. In anexemplary embodiment, MEMS 55 is a 1 mm diameter mirror LV VOA MEMS chipmanufactured by Precisely Microtechnology Corp.

Switching module 53 also includes an optical power element in the formof a spherical lens 57 having a focal length f. Lens 57 is positioned ata distance f from both micro-lens array 11 and MEMS 55 to convert theangular correction of the beam switching trajectories to a correspondingspatial offset in the switching dimension. The electronic control ofMEMS 55 provides an LCOS independent alignment adjustment to center anaxis of symmetry of WSS 1 about a nominal position in the switchingplane. Electronic control of MEMS 55 is provided from active correctionunit 35 through a control line 51, as shown in FIG. 2.

Referring again to FIG. 2, WSS 1 includes a processor 58 for performingsteps of the above calibration procedure. In other embodiments,processor is controlled either wirelessly or through a communicationscable by an external computer (not shown). Referring now to FIG. 5,there is illustrated the primary steps of a method 59 for calibratingoptical beams in WSS 1. At step 61, processor 58 controls laser 39 toproject optical monitor beam 29 through WSS 1 and onto reference region41 of LCOS 17. Beam 29 is reflected from LCOS 17 along a predeterminedmonitor trajectory based on the programmed states of cells withinreference region 41. At step 63, monitor beam 29 reflected from LCOS 17is detected by detector 45. In response to the detected signal, at step65, monitor 27 provides a calibration signal to active correction unit35. In response to receiving the calibration signal, at step 67, unit 35applies a correction to one or more of the switching trajectoriesthrough corresponding control signals to one or more of LCOS 17, grism15 and MEMS 55.

In one embodiment, active correction unit 35 is configured to adjust thewavelength channel alignment in both the switching plane and thedispersion plane, as well as reduce the level of optical flickerexperienced in LCOS 17.

At the process level, the general procedure of method 59 is performed asmethod 69 as illustrated in FIG. 6. At step 71 an initializationprocedure is performed. At step 73, a MEMS mirror alignment procedure isperformed which sets MEMS 55 to an optimum tilt angle for maximizing thedetected power of monitor beam 29. At step 75, an LCOS wavelengthreferencing procedure is performed. At optional step 77, a grismtemperature control procedure is performed. Step 77 is generally onlyperformed if additional adjustment is needed in the dispersion planeafter the LCOS calibration.

Referring to FIG. 7 there is illustrated an exemplary state transitiondiagram describing the high level flow between the stages describedabove. After a reset power cycle or master reset occurs, initializationstage (step 71) is performed. This initialization procedure is performedagain if the mirror alignment is lost. If a soft reset occurs, theinitialization procedure is not required.

Exemplary subroutines for the initialization, MEMS mirror alignment andLCOS wavelength referencing steps are outlined below.

Overview of the Initialization Procedure

The initialization stage has the following goals:

-   -   Determine a minimum optical output power for laser 39 required        to perform all subsequent operations of MEMS mirror alignment        and LCOS wavelength referencing.    -   Perform a linear scan of the MEMS mirror angle to determine an        initial optimum drive voltage (and corresponding optimum tilt        angle) that maximize the reflected light.    -   Characterize and normalize power readings for the Fabry-Perot        etalon's transmitted peaks.    -   Characterize and normalize power readings for the MEMS mirror        feedback.

To achieve these goals, initialization procedure 79 of FIG. 8 isperformed. At step 81, the drive current of DFB laser 39 is adjusted toproduce a predetermined threshold output optical power and lock thelaser drive current. By way of example, in this step the output powermay be set to an initial power output of about 1 mW. At step 83, thetemperature of DFB laser 39 is varied to tune the laser frequency toscan across one of the resonant peaks of the Fabry-Perot etalon. At step85, the out-of-band reference region 41 in LCOS 17 is allocated to serveas a pass-band in the frequency of the laser light. Here, out-of-bandmeans a frequency range separate from the frequencies of the wavelengthchannels being switched in WSS 1. The pass-band of reference region 41is centered on the laser frequency and extends on each side of the laserfrequency by a predetermined bandwidth. At step 87, MEMS 55 is drivenwith a number of drive voltages to tilt the mirror at various angles.During this process, the reflected optical power of laser 39 is detectedto calibrate the maximum and minimum powers at the various MEMS mirrorangles. The mirror angle is initially set at the angle where a maximumoptical power is detected. At step 89, the drive voltages of the variouscells within reference region 41 of LCOS 17 are varied and the reflectedoptical power of laser 39 is measured to calibrate the reference region.Specific levels of attenuation can be applied to produce a desired powerlevel.

At the various steps in procedure 79, if the output power of DFB laser39 is determined to be too low or too high, the output power isincreased or decreased accordingly.

Overview of the MEMS Mirror Alignment

The tilt angle of MEMS 55 controls the position of the beams in theswitching dimension. The angular correction provided by MEMS 55translates to a simultaneous spatial offset to each of the wavelengthchannel optical beams at the output ports. The primary aim of MEMS 55 isto compensate for thermal changes that affect the optical design of WSS1.

Controller 43 performs a control algorithm for selectively adjusting theangle of MEMS 55 in the switching dimension such that the power of beam29 sensed by detector 45 is maximized. This corresponds to the mirrorangle that optimally corrects for thermal changes in the optics of WSS1. Determination of an optimal mirror angle is made through controller43 performing an iterative nonlinear optimization procedure 91 asillustrated in FIG. 9. In another embodiment, an iterative adaptivecontrol procedure is performed to determine the optimal mirror angle.

At initial step 93, MEMS 55 is set to an initial tilt angle in theswitching plane by driving MEMS 55 at a first drive voltage (V). Theinitial tilt angle is typically the angle set during step 87 ofinitialization process 79. The relationship between the mirror angle andthe required voltage is nonlinear and is predetermined so that mirrorangle is related to drive voltage through a look-up table. Monitor beam29 is then projected from laser 39 onto MEMS 55, through grism 15 andreflected back from LCOS 17 to detector. At step 95, the optical power(P) of monitor beam 29 is detected at detector 45. As the mirror angledeviates from the optimal angle, the optical power (P) detected bydetector 45 falls off nonlinearly. At step 97, controller 43 comparesthe current drive voltage (V_(t)) with the previous drive voltage(V_(t-1)) and the current optical power (P_(t)) with the previousoptical power (P_(t-1)). This comparison of the power and drive voltageproduces four possible outcomes which are progressed by deciding whetherto (a) increase the drive voltage, (b) decrease the drive voltage or (c)define the current drive voltage as the drive voltage that produces theoptimum tilt angle.

Upon comparison of consecutive power measurements, if the currentoptical power is determined to be greater than the previous opticalpower (P_(t)−P_(t-1)>0), then at step 99 the mirror is determined to betilting in the direction towards the optimal tilt angle. If the currentoptical power is determined to be less than the previous optical power(P_(t)−P_(t-1)<0) then at step 99 the mirror is determined to be tiltingin the direction away from the optimal tilt angle.

If the voltage is determined to have increased from the previous step(V_(t)−V_(t-1)>0) and the power has increased, then at step 101 thevoltage is again increased to progress closer to a drive voltage thatproduces an optimal tilt angle. Similarly, if the voltage is determinedto have decreased from the previous step (V_(t)−V_(t-1)<0) but the powerhas increased, then at step 101 the voltage is again decreased. If thevoltage is determined to have increased from the previous step but thepower has decreased, then at step 103 the voltage is reduced. Finally,if the voltage is determined to have decreased from the previous stepand the power has decreased, then at step 103 the voltage is increased.The amount of voltage increase or decrease to apply each iteration isproportional to the magnitude of the difference in consecutive powermeasurements. That is, a large difference in power will result in alarger increase or decrease in the drive voltage. Similarly, a smalldifference in power will result in a smaller increase or decrease in thedrive voltage.

Procedure 91 ends at step 105 when an optimum tilt angle is reached. Anoptimum tilt angle is specified as a tilt angle that produces an opticalpower that is equal to or greater than a predefined threshold value,taking into account optical losses. For example, a threshold value foran optimal power might be 1.0 mW. So, if an optical power of 1.1 mW isdetected, then procedure 91 is terminated and the current tilt angle isdefined as an optimal tilt angle. Until the detected power reaches theoptimal power, steps 95 to 103 are repeated iteratively. In analternative embodiment, the optimum tilt angle is determined when themeasured power level drops for voltages either side of a particulardrive voltage.

As the control of MEMS 55 occurs during normal operation of WSS 1, tiltcontrol is performed with a view to maximizing controller stability. Toreduce mechanical dynamics and allow time for oscillation settling,procedure 91 is preferably performed in discrete time rather thancontinuous time. Although this control algorithm provides directcoupling optimization, accurate alignment locking can also be achievedwithout the need to dither the MEMs. To achieve this non optimalalignment of the monitor beam, the LCOS image on either side of therequired alignment is varied and the power measured for each of thesecases. When the powers are balanced for each misalignment then the restof the WSS device is in correct alignment.

Overview of the LCOS Calibration

Each spatially separated wavelength channel is incident onto LCOS 17 ata specific region of cells 19. At LCOS 17, each channel is highlyelongated in the switching plane by the focal power of lens 23.Accordingly, each specific region allocated to a corresponding channelis similarly elongated having a large number of cells 19 extending inthe switching dimension (x-dimension in FIG. 1). During ordinaryoperation of WSS 1, the cells of each channel region are driven with asteering image along the length of the region so as to steer thewavelength channels in the switching dimension between predeterminedinput and output fibers.

Although it is possible to overlap these steering images with furthercalibration images in the switching plane, this adds complexity to thealgorithms. By tuning MEMS 55 using the procedure described above, theneed to provide calibration in the switching plane with LCOS 17 isgreatly reduced. Rather, calibration using the LCOS 17 can be focused onthe dispersion plane while maintaining the standard steering patterns inthe switching plane.

Through appropriate calibration of LCOS pixels 19 (and temperaturecontrol of grism 15), locked calibration system 25 also providescorrective channel position control in the dispersion plane. To achievethis compensation, laser 39 is used as a reference to determine amapping of wavelength-to-pixel during the lifetime of the WSS. Thiswavelength-to-pixel mapping is then used by the optical algorithms tocalibrate the images drawn in the LCOS to dynamically compensate forwavelength shifts.

Referring now to FIG. 10, there is illustrated a procedure 105 fordetermining an LCOS calibration image. Procedure 105 occurs when MEMS 55is locked at the optimum tilt angle determined from procedure 91 of FIG.9. The primary purpose of using LCOS calibration images is to compensatefor thermal gradients in the optics of WSS 1 and aging effects thatcause wavelength shift errors against the nominal ITU grid. These errorsare compensated for by dynamically adjusting the LCOS cell states basedon a known reference signal (monitor beam 29 from laser 39).

At step 107, a calibration procedure is performed. This includes thefollowing sub-steps:

-   -   Tuning the frequency of laser 39 to scan across one of the        Fabry-Perot etalon's transmission peaks and normalizing the        power detected at detector 45 to a common scale of 0% to 100%.        This calibrates for changes related to instabilities in the        laser output power and other optical components.    -   Tuning the laser frequency to a roll-off of a peak in the        Fabry-Perot etalon profile, where the maximum power sensitivity        is found.    -   Determining the frequency of the laser light from the detected        power reading and measuring the Fabry-Perot etalon's current        temperature.    -   Set the cells of reference region 41 to 0 dB attenuation.        Monitor and record the power detected by PD3. This is the 100%        power that can be detected during the wavelength referencing        stage (ceiling). This calibrates away changes related to aging        of laser output power and PD3.    -   Set the cells of reference region 41 to a blocking mode (maximum        attenuation) and monitor and record the power detected. This        sub-step determines the 0% power detected or noise floor. This        calibrates for changes related to aging of laser output power.

At step 109, the states of liquid crystal cells within reference region41 are modified in the dispersive plane so as to create an effectivespectral notch filter and the received power is detected at detector 45.The notch filter bandwidth is centered on the laser frequency. In anexemplary embodiment, the notch filter has a bandwidth of about 25 GHzand the lower 12.5 GHz is given a 180° phase shift with respect to theupper 12.5 GHz.

At step 111, the center frequency of the notch filter created by thecells of reference region 41 is modified in predefined increments untilthe detected power is minimized. This represents a position where thecenter frequency of the notch filter coincides with the laser's centerfrequency. In an exemplary embodiment the notch filter center frequencyis modified in increments of 0.1 GHz. At step 113, the center frequencyof the notch filter is recorded and the known center frequency of thelaser light is used to determine a correction for correcting the WSS anywavelength registration error against the nominal ITU grid. At step 115,this correction is applied to regions of the LCOS corresponding to otherwavelength channels. This includes modifying the state of the liquidcrystal cells across a wavelength channel while still maintaining thecorrect switching state for switching the channel to a predefined outputfiber. In some cases, different corrections are required to differentwavelengths. These can be calculated by varying the frequency of laser39 over different calibration cycles.

In one embodiment, a separate calibration procedure is performed withLCOS 17 so as to reduce the optical flicker experienced on LCOS 17.Optical flicker is detected as a ripple in power of the detected monitorbeam 29 and is more easily detected by certain phase images produced byLCOS 17. As an example, by applying a ramp image function to referenceregion 41 with a greater than 2π phase change, the detected monitor beamis sensitive to the power ripple. By iteratively adjusting the commonvoltage applied to LCOS 17 until the ripple is minimized, the opticalflicker can be substantially reduced.

Use of monitor 27 is able to be time multiplexed for separate use indetermining the MEMS mirror angle to calibrate in the switching plane,calibrating LCOS 17 for control in the dispersion plane and detectingand reducing optical flicker.

Higher Order Mode Suppression

In dynamically routing optical beams through WSS 1, the steeringfunctions applied to LCOS 17 inherently also couple diffraction ordershigher than the zero order through the optical system. If these higherorders are not sufficiently suppressed, they can couple to nearby fibersand give rise to optical interference in the form of cross-talk. Twoparticular higher diffraction orders that contribute most to cross-talkare the −1 diffraction order and the +2 diffraction order. By applyingcertain attenuation patterns on LCOS 17, it is possible to efficientlysuppress the −1 order robustly over a wide range of operatingconditions. Suppression of the +2 order, however, is more difficult dueprimarily to the presence of unwanted double-pass orders which are phasesensitive to optical elements such as the top glass thickness of LCOS17. The tight control of the beam positions provided by the abovedescribed locked calibration system provides a solution for allowing thegreater suppression of the +2 diffraction order mode.

Referring to FIG. 11, there is illustrated a schematic front view of afiber v-groove array 117 for maintaining up to twenty optical fibers,e.g. 119. The fibers are disposed about a central axis 121. Axis 121 isdefined as a point of symmetry of an optical trajectory between inputand output fibers when LCOS 17 provides no steering in the switchingplane. The position of axis 121 is controlled by the tilt angle of MEMS55.

V-groove array 117 includes a lower mounting portion 123 having aplurality of spaced apart v-shaped grooves 125 for receiving opticalfibers. V-groove array 117 also includes an upper clamping portion 127for securely engaging the optical fibers in their respective v-shapedgrooves. Mounting portion 123 and clamping portion 127 are selectivelyreleasably engagable to secure the fibers in their respective grooves.

The majority of the fibers are equally spaced 250 μm apart. However,arbitrary pairs of fibers 127 and 129 on each side of axis 121 arespaced apart by 375 μm. In other embodiments, other fiber spacings areused. The fibers are disposed in an array about axis 121 and begin at adistance of 500 μm from axis 121. This particular arrangement of fibersensures that, with proper beam control using locked calibration system25, the +2 diffraction orders of wavelength channels directed to theadd/drop ports fall between the respective fibers and do notsignificantly contribute to cross-talk.

Locked calibration system 25 is able to monitor and adjust the relativeposition, in the switching plane, of the diffraction orders for specificwavelength channels by applying a background ramp function to cells ofLCOS 17 corresponding to those specific channels. This ramp function isapplied in conjunction with the current switching state to apply acorrection or calibration to the optical beam trajectories. By suitablecalibration using MEMS 55 and LCOS 17, locked calibration system 25 isable to collocate the various diffraction orders for the variouswavelength channels so that unwanted orders are incident between thefiber ports.

Conclusions

It will be appreciated that embodiments of the present invention providea calibration system for a wavelength selective switch.

The present invention provides a technique for dynamically adjusting thealignment of optical beams within the optical train of a WSS device tocompensate for aberrations and variations to the beams. The compensationis provided by the dynamic calibration of the wavelength-to-pixelmapping during operation of the WSS. In order to achieve thiscompensation, a wavelength reference laser is used to find mapping ofwavelength-to-pixel during the lifetime of the WSS. Thiswavelength-to-pixel mapping is then used by optical algorithms tocalibrate the images drawn in the LCOS to dynamically compensate forwavelength shifts. In some embodiments, the channel registration erroris contained to less than about 1.5 GHz. In the case of dual source typeWSS devices, the described calibration techniques can also account forimperfections between the two sources.

The combination of fiber array symmetry and the described calibrationtechniques provides for controlling the position of all diffractionorders and, in particular, ensuring that no direct coupling goes to +2diffraction orders which suffer from deleterious effects in the presenceof unwanted low level reflections.

The more accurate stability provided by the above described activefeedback locked calibration system allows the structural thermal andmechanical stability requirements of the wavelength selective switch tobe relaxed. In some embodiments, only the LCOS device is temperaturecontrolled and aberrations from the remaining optical elements arecompensated actively using the locked calibration system. By relaxingthe temperature control of the WSS, the thickness of the mountingsubstrate can be reduced substantially. This inherently reduces theoverall profile of the packaged device and reduces material costs.

Although discussed in relation to a specific WSS device 1, it will beappreciated that locked calibration system 25 is also applicable toother types of WSS.

Interpretation

Throughout this specification, use of the term “element” is intended tomean either a single unitary component or a collection of componentsthat combine to perform a specific function or purpose.

Throughout this specification, use of the term “orthogonal” is used torefer to a 90° difference in orientation when expressed in a Jonesvector format or in a Cartesian coordinate system. Similarly, referenceto a 90° rotation is interpreted to mean a rotation into an orthogonalstate.

Unless specifically stated otherwise, as apparent from the followingdiscussions, it is appreciated that throughout the specificationdiscussions utilizing terms such as “processing,” “computing,”“calculating,” “determining”, analyzing” or the like, refer to theaction and/or processes of a computer or computing system, or similarelectronic computing device, that manipulate and/or transform datarepresented as physical, such as electronic, quantities into other datasimilarly represented as physical quantities.

In a similar manner, the term “processor” may refer to any device orportion of a device that processes electronic data, e.g., from registersand/or memory to transform that electronic data into other electronicdata that, e.g., may be stored in registers and/or memory. A “computer”or a “computing machine” or a “computing platform” may include one ormore processors.

The methodologies and processes described herein are, in one embodiment,performable by one or more processors that accept computer-readable(also called machine-readable) code containing a set of instructionsthat when executed by one or more of the processors carry out at leastone of the methods described herein. Any processor capable of executinga set of instructions (sequential or otherwise) that specify actions tobe taken are included. Thus, one example is a typical processing systemthat includes one or more processors. Each processor may include one ormore of a CPU, a graphics processing unit, and a programmable DSP unit.The processing system further may include a memory subsystem includingmain RAM and/or a static RAM, and/or ROM. A bus subsystem may beincluded for communicating between the components. The processing systemfurther may be a distributed processing system with processors coupledby a network. If the processing system requires a display, such adisplay may be included, e.g., a liquid crystal display (LCD) or acathode ray tube (CRT) display. If manual data entry is required, theprocessing system also includes an input device such as one or more ofan alphanumeric input unit such as a keyboard, a pointing control devicesuch as a mouse, and so forth. The term memory unit as used herein, ifclear from the context and unless explicitly stated otherwise, alsoencompasses a storage system such as a disk drive unit. The processingsystem in some configurations may include a sound output device, and anetwork interface device. The memory subsystem thus includes acomputer-readable carrier medium that carries computer-readable code(e.g., software) including a set of instructions to cause performing,when executed by one or more processors, one of more of the methodsdescribed herein. Note that when the method includes several elements,e.g., several steps, no ordering of such elements is implied, unlessspecifically stated. The software may reside in the hard disk, or mayalso reside, completely or at least partially, within the RAM and/orwithin the processor during execution thereof by the computer system.Thus, the memory and the processor also constitute computer-readablecarrier medium carrying computer-readable code.

Furthermore, a computer-readable carrier medium may form, or be includedin a computer program product.

In alternative embodiments, the one or more processors operate as astandalone device or may be connected, e.g., networked to otherprocessor(s), in a networked deployment, the one or more processors mayoperate in the capacity of a server or a user machine in server-usernetwork environment, or as a peer machine in a peer-to-peer ordistributed network environment. The one or more processors may form apersonal computer (PC), a tablet PC, a set-top box (STB), a PersonalDigital Assistant (PDA), a cellular telephone, a web appliance, anetwork router, switch or bridge, or any machine capable of executing aset of instructions (sequential or otherwise) that specify actions to betaken by that machine.

Note that while diagrams only show a single processor that carries thecomputer-readable code, those in the art will understand that many ofthe components described above are included, but not explicitly shown ordescribed in order not to obscure the inventive aspect. For example,while only a single machine is illustrated, the term “machine” shallalso be taken to include any collection of machines that individually orjointly execute a set (or multiple sets) of instructions to perform anyone or more of the methodologies discussed herein.

Thus, one embodiment of each of the methods described herein is in theform of a computer-readable carrier medium carrying a set ofinstructions, e.g., a computer program that is for execution on one ormore processors, e.g., one or more processors that are part of webserver arrangement. Thus, as will be appreciated by those skilled in theart, embodiments of the present invention may be embodied as a method,an apparatus such as a special purpose apparatus, an apparatus such as adata processing system, or a computer-readable carrier medium, e.g., acomputer program product. The computer-readable carrier medium carriescomputer readable code including a set of instructions that whenexecuted on one or more processors cause the processor or processors toimplement a method. Accordingly, aspects of the present invention maytake the form of a method, an entirely hardware embodiment, an entirelysoftware embodiment or an embodiment combining software and hardwareaspects. Furthermore, the present invention may take the form of carriermedium (e.g., a computer program product on a computer-readable storagemedium) carrying computer-readable program code embodied in the medium.

The software may further be transmitted or received over a network via anetwork interface device. While the carrier medium is shown in anexemplary embodiment to be a single medium, the term “carrier medium”should be taken to include a single medium or multiple media (e.g., acentralized or distributed database, and/or associated caches andservers) that store the one or more sets of instructions. The term“carrier medium” shall also be taken to include any medium that iscapable of storing, encoding or carrying a set of instructions forexecution by one or more of the processors and that cause the one ormore processors to perform any one or more of the methodologies of thepresent invention. A carrier medium may take many forms, including butnot limited to, non-volatile media, volatile media, and transmissionmedia. Non-volatile media includes, for example, optical, magneticdisks, and magneto-optical disks. Volatile media includes dynamicmemory, such as main memory. Transmission media includes coaxial cables,copper wire and fiber optics, including the wires that comprise a bussubsystem. Transmission media also may also take the form of acoustic orlight waves, such as those generated during radio wave and infrared datacommunications. For example, the term “carrier medium” shall accordinglybe taken to include, but not be limited to, solid-state memories, acomputer product embodied in optical and magnetic media; a mediumbearing a propagated signal detectable by at least one processor or oneor more processors and representing a set of instructions that, whenexecuted, implement a method; and a transmission medium in a networkbearing a propagated signal detectable by at least one processor of theone or more processors and representing the set of instructions.

It will be understood that the steps of methods discussed are performedin one embodiment by an appropriate processor (or processors) of aprocessing (i.e., computer) system executing instructions(computer-readable code) stored in storage. It will also be understoodthat the invention is not limited to any particular implementation orprogramming technique and that the invention may be implemented usingany appropriate techniques for implementing the functionality describedherein. The invention is not limited to any particular programminglanguage or operating system.

Reference throughout this specification to “one embodiment”, “someembodiments” or “an embodiment” means that a particular feature,structure or characteristic described in connection with the embodimentis included in at least one embodiment of the present disclosure. Thus,appearances of the phrases “in one embodiment”, “in some embodiments” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures or characteristics may be combined inany suitable manner, as would be apparent to one of ordinary skill inthe art from this disclosure, in one or more embodiments.

As used herein, unless otherwise specified the use of the ordinaladjectives “first”, “second”, “third”, etc., to describe a commonobject, merely indicate that different instances of like objects arebeing referred to, and are not intended to imply that the objects sodescribed must be in a given sequence, either temporally, spatially, inranking, or in any other manner.

In the claims below and the description herein, any one of the termscomprising, comprised of or which comprises is an open term that meansincluding at least the elements/features that follow, but not excludingothers. Thus, the term comprising, when used in the claims, should notbe interpreted as being limitative to the means or elements or stepslisted thereafter. For example, the scope of the expression a devicecomprising A and B should not be limited to devices consisting only ofelements A and B. Any one of the terms including or which includes orthat includes as used herein is also an open term that also meansincluding at least the elements/features that follow the term, but notexcluding others. Thus, including is synonymous with and meanscomprising.

It should be appreciated that in the above description of exemplaryembodiments of the disclosure, various features of the disclosure aresometimes grouped together in a single embodiment, Fig., or descriptionthereof for the purpose of streamlining the disclosure and aiding in theunderstanding of one or more of the various inventive aspects. Thismethod of disclosure, however, is not to be interpreted as reflecting anintention that the claims require more features than are expresslyrecited in each claim. Rather, as the following claims reflect,inventive aspects lie in less than all features of a single foregoingdisclosed embodiment. Thus, the claims following the DetailedDescription are hereby expressly incorporated into this DetailedDescription, with each claim standing on its own as a separateembodiment of this disclosure.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe disclosure, and form different embodiments, as would be understoodby those skilled in the art. For example, in the following claims, anyof the claimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the disclosure maybe practiced without these specific details. In other instances,well-known methods, structures and techniques have not been shown indetail in order not to obscure an understanding of this description.

Similarly, it is to be noticed that the term coupled, when used in theclaims, should not be interpreted as being limited to direct connectionsonly. The terms “coupled” and “connected,” along with their derivatives,may be used. It should be understood that these terms are not intendedas synonyms for each other. Thus, the scope of the expression a device Acoupled to a device B should not be limited to devices or systemswherein an output of device A is directly connected to an input ofdevice B. It means that there exists a path between an output of A andan input of B which may be a path including other devices or means.“Coupled” may mean that two or more elements are either in directphysical, electrical or optical contact, or that two or more elementsare not in direct contact with each other but yet still co-operate orinteract with each other.

Thus, while there has been described what are believed to be thepreferred embodiments of the disclosure, those skilled in the art willrecognize that other and further modifications may be made theretowithout departing from the spirit of the disclosure, and it is intendedto claim all such changes and modifications as fall within the scope ofthe disclosure. For example, any formulas given above are merelyrepresentative of procedures that may be used. Functionality may beadded or deleted from the block diagrams and operations may beinterchanged among functional blocks. Steps may be added or deleted tomethods described within the scope of the present disclosure.

I claim:
 1. A calibration system for a wavelength selective switch, theswitch being adapted for dynamically switching optical beams alongrespective trajectories between input and output ports disposed in anarray using a reconfigurable spatial light modulator device, the systemincluding: a monitor for the projection of an optical monitor beamthrough at least a portion of the wavelength selective switch onto apredefined region of the spatial light modulator, the monitor furtherincluding a controller for electrically controlling cells within thepredefined region to selectively direct the optical monitor beam along amonitoring trajectory relative to a switching trajectory, the monitorfurther providing for a detection of the optical monitor beam reflectedfrom the spatial light modulator device and, in response, providing acalibration signal; and an active correction unit responsive to thecalibration signal for applying a correction to one or more of thetrajectories.
 2. The calibration system according to claim 1 wherein thesame correction is applied simultaneously to each of the respectivetrajectories.
 3. The calibration system according to claim 1 wherein thecorrection includes a predefined spatial offset of the optical beams atthe output ports.
 4. The calibration system according to claim 3 whereinthe spatial offset of the optical beams is in a switching dimensionbeing a direction of switching by the spatial light modulator device. 5.The calibration system according to claim 1 wherein the activecorrection unit provides control to a beam switching module including:an electrically controllable mirror tiltable at a number of predefinedangles; and an optical power element having a focal length f and beingpositioned at a distance f from both the array of input and output portsand the mirror to convert an angular correction of the trajectories to acorresponding spatial offset in a dimension of the array.
 6. Thecalibration system according to claim 1 wherein the active correctionunit includes a control for the spatial light modulator for selectivelymodifying a background slope image while maintaining the currentswitching state.
 7. The calibration system according to claim 1 whereinthe monitor beam is projected from a first monitor port in the array andreceived in a second monitor port in the array.
 8. The calibrationsystem according to claim 7 wherein the first and second monitor portsare the same port.
 9. The calibration system according to claim 1wherein the monitor includes: a light source for directing the monitorbeam onto the predetermined region; the controller for electricallycontrolling cells within the predefined region to selectively direct themonitor beam along a monitoring trajectory relative to a switchingtrajectory; and a detector for detecting the optical power of the beamdirected along the monitoring trajectory.
 10. The calibration systemaccording to claim 9 wherein the light source produces a monitor beamhaving a predetermined wavelength.
 11. The calibration system accordingto claim 10 wherein the light source is a wavelength locked laser. 12.The calibration system according to claim 11 wherein the light sourceincludes a tunable element for selectively defining the predeterminedwavelength of the monitor beam.
 13. The calibration system according toclaim 1 wherein the predefined region is located in a peripheral regionof the spatial light modulator.
 14. The calibration system according toclaim 1 wherein the spatial light modulator is an LCOS device.
 15. Thecalibration system according to claim 1 wherein the active correctionunit applies the correction to the one or more trajectories whilemaintaining a constant switching state in the spatial light modulator.16. A monitor device for a wavelength selective switch, the switch beingadapted for dynamically switching optical beams along respectivetrajectories between input and output ports disposed in an array using areconfigurable liquid crystal spatial light modulator device, themonitor including: a monitor for monitoring predeterminedcharacteristics of one or more of the optical beams, the monitorcomprising a light source for directing an optical monitor beam onto apredetermined region, a controller for electrically controlling cellswithin the predefined region to selectively direct the optical monitorbeam along a monitoring trajectory relative to a switching trajectory,and a detector for detecting an optical power of the optical monitorbeam directed along the monitoring trajectory; and an active feedbackcontroller responsive to the monitor for simultaneously correcting formore than one of a beam alignment, wavelength position and liquidcrystal optical flicker.
 17. A calibration system for monitoring theoperation of a pixelated optical phased array structure, the systemincluding: a first laser for projecting a reference signal onto aportion of the pixelated optical phased array structure, a sensor formonitoring a return reference signal from a first portion of thepixelated optical phased array structure so as to provide a sensedreturn signal, a control system for adjustment of a position of phasepatterns on a second portion of the pixelated optical phased arraystructure based on the sensed return signal.
 18. The calibration systemaccording to claim 17 wherein the pixelated optical phased arraystructure forms part of an optical system having a series ofinput/output ports, an optical dispersion device and optical powerelements and wherein an input signal from the input/output ports isprojected through each of the optical dispersion device and opticalpower elements.
 19. The calibration system according to claim 17 whereinthe input signal is incident onto the pixelated optical phased arraystructure in a first region and the reference signal is incident ontothe pixelated optical phased array structure in a second region separateto the first region.
 20. The calibration system according to claim 17wherein the first laser is tunable.